Rotor and gear assembly for rotary mechanisms

ABSTRACT

An improved rotor and gear assembly for rotary mechanisms of the trochoidal type, in which a rotor having a central bore is mounted for rotation on a shaft, with a bearing between the rotor bore and the journal portion of the shaft. An internally toothed ring gear is secured to a side face of the rotor for engagement with a stationary spur gear to maintain phasing between the rotor and its trochoidal housing during the planetary and rotary motion of the rotor within the housing. The ring gear is mounted on the rotor in such a manner as to maintain concentricity therewith while permitting differences in thermal expansion between the materials of the rotor and the gear, and without imposing stress on the mounting means or the bearing without causing distortion in the gear or the rotor.

United States Patent 1191 Poehiman Aug. 20, 1974 ROTOR AND GEAR ASSEMBLY FOR ROTARY MECHANISMS Primary lgxaminer(ie J Husasr th Assistant xamineronar mi [75] Invsmor' a Poehlman west Bend Attorney, Agent, or Firm-Raymond P. Wallace [73] Assignee: Outboard Marine Corporation, [57] ABSTRACT Waukegan, An improved rotor and gear assembly for rotary [22] Filed; June 25, 1973 mechanisms of the trochoidal type, in which a rotor having a central bore is mounted for rotation on a PP N05 373,516 shaft, with a bearing between the rotor bore and the journal portion of the shaft. An internally toothed ring 52 US. Cl 418/61 A gear is secured to a Side face of the rotor for engage- [51 1m. (:1. F04C 1/02 ment with a Stationary Spur gear to maintain Phasing 58 Field of Search 418/61 A' 29/l56.4 R- between the rotor and its trochoidal housing during 74/4,] 1 432 the planetary and rotary motion of the rotor within the housing. The ring gear is mounted on the rotor in such [56] References Cited a manner as to maintain concentricity therewith while permitting differences in thermal expansion between UNITED STATES PATENTS the materials of the rotor and the gear, and without 3,297,240 1/1967 Tatsutomi 418/61 A imposing Stress on the mounting means or the bearing :52 2 without causing distortion in the gear or the rotor. 316551302 4/1972 Hermes et al .418/61 A 13 Claims, 8 Drawing Figures PAIENImauczomu sum in? 2 ROTOR AND GEAR ASSEMBLY FOR ROTARY MECHANISMS BACKGROUND This invention relates to rotary mechanisms of the trochoidal type for pumps, compressors, internal combustion engines, and fluid motors, and more particularly to a rotor and gear assembly for such mechanisms.

In such mechanisms of the prior art the bearing on the shaft journal has sometimes been a barrel extension of the ring gear, as in U.S. Pat. No. 3,111,261. In that patent the ring gear was radially splined to the rotor to prevent relative rotation between the two parts and still allow differences in thermal growth. The gear was retained axially by a ringnut at the opposite end of the bearing sleeve. Since thermal growth of the rotor would free the bearing sleeve from the rotor bore with possible excess clearance therebetween, the sleeve was surrounded by a second bearing sleeve with a severe shrink fit in the rotor, so that clearances developing would be divided between the shaft journal and the adjacent inner bearing, and between the inner bearing and the outer bearing. Such an arrangement requires a good deal of precision in manufacture and fitting at assembly, and is very expensive to produce and install. Further, although differential radial growth of the parts was accommodated, the axial retention of the bearings by the ringnut did not adequately accommodate differential axial expansion.

A generally similar arrangement is disclosed in U.S. Pat. No. 3,230,789 wherein a single bearing is used, having splines extending radially therefrom with the rotor cast onto the outer circumference of the bearing sleeve and engaging the splines. The ring gear is bolted to the splines of the bearing. Although thisconstruction may be somewhat cheaper than the previous one, it is nevertheless an expensive item, requiring that the radially extending splines be highly finished before casting the rotor metal on them, since slippage arising-from differential expansion must be along the spline surfaces. Further, the attachment of the gear byfirm bolting to the splines of the bearing restricts thermal growth of the bearing.

U.S. Pat. No. 3,383,936 is a further example of expensive splining between the rotor and the gear to prevent rotation of the gear in respect to the rotor, while allowing radial differential expansion. The device shown therein also contains firm axial attachments between the gear and rotor so that axial expansion imposes a strain on the parts.

In U.S. Pat. No. 3,400,604 the gear is restrained from rotation relative to the rotor by dowel pins having a tight fit in both parts, and it is stated that radial growth of the rotor will be transferred to the gear by the equally spaced dowels and thus maintain concentricity. This assumption has been found to be incorrect, however, since the gear, being of more rigid material than the rotor and having a lower coefficient ofv thermal expansion, restricts rotor growth and causes distortions therein and strain on the dowel pins.

SUMMARY The present invention provides a rotor and gear assembly for rotary mechanisms of the trochoidal type, wherein the gear is secured against rotation relative to the rotor and against transverse motion relative thereto, provision is made for thermal expansion both radially and axially, concentricity is maintained, distortion of parts is obviated, and thermal strain on the attaching means is prevented. These advantages are accomplished by mounting the gear on pins or dowels which have a tight fit in the rotor body, and a tight fit in the gear in the circumferential direction, but allow thermal growth in radial directions. Axial growth of both the rotor and the gear is provided for, and axial movement of the gear is restrained in one direction by seating it on the rotor hub, and in the other direction by detent means allowing sufficient clearance for the anticipated dimensional changes.

it is therefore an object of the invention to provide a rotor and ring gear assembly for trochoidal rotary mechanisms allowing differential thermal growth of the parts without strain or distortion.

It is another object to provide such an assembly wherein the gear maintains concentricity with the rotor.

A further object is to provide such a rotor and gear assembly having a bearing for the shaft journal, wherein the bearing is not subject to constriction in the radial or axial directions by reason of the gear mount- Still another object is to provide such an assembly wherein the material of the rotor body and the material of the ring gear have different coefficients of thermal expansion.

It is a further object to provide a rotor and ring gear assembly wherein the rotor body is formed of a relatively lightweight metal.

Other objects and advantages will become apparent on reading the following specification in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an elevation looking in the axial direction of a rotor and ring gear assembly for a trochoidal mechanism having a two-lobed peripheral housing;

FIG. 2 is a cross-sectional elevation taken on the line 22 of FIG. 1;

FIG. 3 is an enlarged fragmentary view looking in the direction of line 3 -3 of FIG. 2;

FIG. 4 is an enlarged detail portion of FIG. 2;

FIG. 5 is a view similar to FIG. 1 of another embodiment;

FIG. 6 is an elevation partly in cross-section along the line 6-6 of FIG. 5;

FIG. 7 is an enlarged fragmentary view looking in the direction of line 7-7 of FIG. 6; and

FIG. 8 is an enlarged detail portion of FIG. 5.

DESCRIPTION OF A PREFERRED EMBODIMENT FIGS. 1 and 2 show a rotor 11 of generally triangular profile having three convexly arcuate working faces 12, which is suitable for use in a trochoidal mechanism having a two-lobed housing. The invention will be described in terms of such a triangular rotor, but is it to be understood that the rotor assembly of this invention is also applicable to mechanisms of other trochoidal design, such as one-lobed, three-lobed, etc., wherein the rotor has a profile differing from that shown. The main body of the rotor is hollow and comprises a peripheral wall portion 13, a pair of parallel side walls 14 and 16,

and a hub portion 17 which is joined to the outer portion 13 by appropriate ribs or webs 18. Each working face 12 has therein a recess 19 of a size and shape selected appropriately for the intended use of the mechanism, or which in some cases may be omitted, as for example in pumps or compressors. The usual sealing means at the rotor apexes and side walls are omitted from the showing, as not being necessary to an understanding of the invention.

Side wall 14 has a central circular aperture 21 thereinand side wall 16 has an aperture 22, the apertures serving for installation of internal parts at assembly and for flow of lubricating and cooling fluids during operation. Hub 17 has a bore 23 therethrough on the rotational axis, with a bearing 24 installed therein. The bearing may be a sleeve type or a roller bearing or other convenient type, and in the assembled rotary mechanism will be mounted on an eccentric portion 26 of a rotatable shaft 27.

As is conventional in trochoidal rotary mechanisms, the rotor bears a ring gear 28 which in operation will be in mesh with a spur gear (not shown) home by a wall of the housing of the mechanism, such gears serving to index the rotation of the rotor in proper registration with the trochoidal housing within which it turns. The hub 17 has one end face 29 machined flat and perpendicular to the rotational axis, against which flat face the ring gear 28 is mounted, coaxial with the rotor body and the bearing 24, and therefore also with the axis of the shaft eccentric portion 26. As shown, gear 28 is recessed wholly within the hollow interior of the rotor, but may in some cases be positioned within aperture 21 with its axially outer face flush with the outer face of side wall 14, or the gear may even have some portion projecting slightly beyond the rotor side wall to run against the end wall of the housing.

In operation of the rotary mechanism the rotor may become considerably heated. This is particularly true when the mechanism is an internal combustion engine, but also occurs with pumps used for pumping hot fluids, with compressors when the gas is heated by the effect of compression, and with fluid motors which are driven by hot fluids. The material of the rotor is therefore subject to thermal expansion during operation, followed by contraction on subsequent cooling. Such rotors are frequently formed of aluminum or one of its alloys in order to save weight and in order that the weight of the rotor shall exert less bearing friction and so consume less power, whether the power is being generated in an engine or applied to a pump or other mechanism. The gear, although it does not transmit torque, is nevertheless subject to ordinary frictional wear, and to intermittent cyclical loading as the rotor moves transversely across bearing clearances owing to the sinusoidal loading of the rotary mechanism. The gear is therefore formed of a suitable gear steel.

This diversity of materials results in the gear and the rotor having different coefficients of thermal expansion, especially in the case of a light weight rotor, since in a general way aluminum and its alloys have about double the thermal expansion of steel. Rotors are sometimes formed of nodular cast iron, and though this material does not have as high a thermal response as aluminum it is nevertheless higher than that of gear steel. Even if the rotor and gear should be made of materials having identical thermal response, the problem of differential expansion would still exist, since the rotor metal is directly exposed to the source of heat, and the interface between the two parts acts as a substantial impediment to heat transfer.

it is important for efficient operation of the rotary mechanism that the rotor and its attached gear should maintain their concentricity, during the initial period of warm-up during steady operation, and during cooling after shutdown. It has been assumed in the prior art that, where the gear was mounted by firm bolting or located by tightly fitting dowels in both parts, or otherwise firmly piloted, the expansion of the rotor hub would be transmitted by the mounting means and compel the gear to expand an equal amount. This has been found not to be the case. Tight dowels have resulted in constraint of the hub on the gear side so that it could not expand so much as the free side, with the result that the bore became tapered and the bearing likewise, with consequent bearing failures. The same defect occurred when the rotor had a flange against which the inner diameter of the gear ring seated. When firm bolting was used, if the bolts were torqued tightly enough to preclude relative transverse movement between the gear and the rotor, the same problem could occur again, and there was no provision for differential axial expansion, again with resultant deformation.

The present invention overcomes these limitations by providing means for attaching the gear to the rotor in such a manner that both the gear and the rotor are free to expand radially and axially without interference or imposing strains on each other, and at the same time the gear is prevented from making any rotation with respect to the rotor and from any motion transverse to the rotor axis, thus retaining concentricity through all operating conditions and at shutdown. These conditions will be maintained whether the gear and the rotor have identical coefficients of thermal expansion, or whether they are different, without regard to whichever part has the greater thermal response.

The plane face 29 of the rotor hub 17 has extending axially therefrom a plurality of cylindrical dowels or pins 31 projecting from the hub face no more than the thickness of the gear 28. The pins may be equiangularly spaced as shown, or may have some other orientation. There must be at least three such pins 31 and ordinarily that is the preferred number, although in some instances there may be more. Pins 31 have a tight fit in the rotor hub, such that they will not become loose when the parts are subjected to the greatest degree of thermal expansion anticipated. A convenient way of securing firm seating of the pins is by an interference fit between the diameters of the pins and the bores in which they are seated. However, the pins may also be secured in the rotor by other conventional means, such as brazing.

Gear 28 is provided with a plurality of apertures 32 slotted in radial directions, the same in number as the number of pins used and identically spaced, into which the pins extend. The circumferential width of slots 32 is such as to provide a tight friction fit with the diameter of the cylindrical pins, and therefore there can be no relative rotation of the gear and the rotor. Likewise, there can be no relative transverse movement, which will always be opposed by the side walls of at least two of the slotted apertures.

In radial directions, however, apertures 32 are elongated sufficiently to allow relative radial movement between the pins and the slots, which permits thermal expansion and contraction without change in the concentricity of the parts. The amount of elongation of apertures 32 in much exaggerated in the drawings for clarity of illustration, but in practice it need not be great. The actual amount of elongation of slots 32 will be chosen in accordance with a given design, with reference to the coefficients of thermal expansion of the rotor metal and the gear metal, the maximum temperature which may be reached at the surface of the rotor, the thermal gradient between the surface and the position of the pins, and the diameter of the circle on which the pins are located. ln engines of moderate size, such as about sixty cubic inches per rotor, the relative movement of the pins within their slots will be only a few thousandths of an inch.

The gear is retained axially (best shown in FIG. 4) by axial detent means comprising a plurality of equiangularly spaced bolts 33 extending through bores in the gear annulus. The bolts 33 each have a smooth cylindrical shank portion 34 with a flat shouldered inner end 36 seated against the flat bottom of a bore 37 in the rotor hub 17, bore 37 having a slightly larger diameter than the bolt shank so that there is no circumferential contact therewith. Coaxial with shank 34 and extend ing inwardly therefrom the bolt has a threaded continuation 38 of smaller diameter,engaged with a tapped hole 39 in hub 17. The threads of bolt portion 38 and hole 39 are in interference engagement, so that when the bolt is torqued down tightly to its shoulder 36 it will not be loosened by thermal expansion.

The axially outer ends of bolt shanks 34 are positioned coaxially in bores 41 through the gear annulus, which bores are also slightly larger than the bolt shanks so that there is no contact. Bores 41 have flat-bottomed coaxial counterbores 42 in the outer gear face, approximately halfway through the thickness of gear 28. The bolts have flat-bottomed heads of smaller diameter than the counterbores and positioned therein, but the length of shanks 34 and the depth of their seating bores 37 are so proportioned that the boltheads do not bear upon the bottoms of counterbores 42, but leave a gap 43 of a few thousandths of an inch. The dimension of gap 43 is proportioned according to the size of the rotary mechanism, the materials of the rotor and gear, and the expected maximum temperature, so that with the maximum thermal expansion anticipated the boltheads will not impose any strain on the gear.

It will therefore be seen that gear 28 is axially linked to the rotor hub so that it cannot be parted therefrom. Although the gear appears to be slightly loose on its axial mounting in the cold state, this is of no importance, since the amount of clearance under the boltheads is minimal. It has been found that in operation the gear will run either against the boltheads or against the rotor hub without play until the temperature reaches a value at which gap 43 has substantially closed.

It will also be apparent that the gear can have no circumferential movement relative to the rotor, owing to the bearing of pins 31 against the sides of the slots 32. Likewise, there can be no relative movement of the gear transverse to the axis, since such transverse movement will always be opposed by at least two of the pins against the sides of their slots, or by a larger number of pins if the total used is more than three. Differential thermal expansion of the parts in any direction is readily possible, but cannot impose any strains or distor1 tions on the gear, the rotor, the mountings, or the bearings, and concentricity cannot be disturbed.

FIGS. 5-8 show anoher embodiment of the same in ventive concept having the same advantages. The rotor here shown is of modified form in order to exemplify the generality of the gear mounting means of the invention and its applicability to the rotors of various rotary mechanisms. Details not necessary to an understanding of the invention have been omitted from the showing.

Rotor 11a is of generally triangular profile with convexly arcuate working faces 12 each having therein a recess 19 as in the previously described embodiment. The main body of the rotor is hollow and comprises a peripheral wall portion 13, parallel side faces 14a and 16a, and a hub portion 17a which is joined to the outer portion 13 by appropriate ribs or webs l8. Hub 17a has a bore 23 therethrough on the rotational axis, with a bearing 24 installed therein. In the assembled rotary mechanism the eccentric portion 26 of a rotatable shaft 27 will be journalled therein.

A ring gear 28a is seated against the flat end face of the rotor hub, coaxial with the rotor body, the bearing, and the eccentric portion 26 of the shaft. The axially inner side of gear 28a is provided with a shouldered extension 46, the outer diameter of which is a close fit to the bore 23 in the hub. The extension 46 is positioned within the bore at the installation of the gear, which insuRes coaxiality, and once the gear is mounted the relationship cannot change. If differential thermal expansion occurs, it will always be greater in the rotor hub than in the gear, so that the hub will only expand radially away from the gear shoulder without imposing any strain on either part.

Differential radial expansion of the two parts without relative transverse or circumferential movement is provided for in the same manner as in the previous embodiment. The hub 17a has protruding axially therefrom a plurality of dowels 31a, which are shown as split tubular dowels. Such split tubes are a convenient means of securing a tight fit in the bores in the hub, since the split tube can be compressed at installation and by its natural resilience will remain firmly fixed even when the hub undergoes thermal expansion. However, it is to be understood that either tubular or solid dowels as previously shown may be used in either embodiment. Although FIG. 5 shows the dowels disposed in three pairs, the invention is not limited to any specific number except that there must be at least three. Further, they need not be equiangularly spaced in either embodiment, since if it should be convenient to position them otherwise the rotor may be appropriately balanced by other means.

The gear 28a is provided with apertures 32, slotted in radial directions and the same in number and spacing as the dowels. The circumferential width of the slots in such as to provide a tight friction fit with the dowel diameter, thus preventing transverse and circumferential movement, while allowing sufficient play in radial directions for the anticipated radial thermal movements.

The detent means for axial retention of the gear comprises a plurality of lugs 47 extending radially outwardly from the circumference of the gear; the lugs 47 may be coplanar with the exposed face of the gear as shown, but are not necessarily so. A like plurality of lugs 48 extend radially inwardly from'the peripheral wall 13 of the rotor body. The axially inward faces of rotor lugs 48 and the axially outward faces of gear lugs 47 are so positioned that lugs 48 overlap lugs 47 with just sufficient axial clearance to allow for any anticipated axial expansion, an amount comparable to that allowed in the previously described embodiment. The axial detent means in either embodiment need not be more than two in number if spaced diametrically opposite, but may be any larger number desired. It is usually convenient to have the same number of axial detents as dowels, spaced intermittently therewith. Further, although the rotor lugs 48 of this embodiment are shown as integral parts of the rotor, they may be separate parts bolted to the rotor or otherwise attached.

The manner of registering the axial detent means when lugs 48 are integral with the rotor is shown in FIG. 8. At assembly of the gear and rotor, the gear is set upon the face of the rotor hub with its barrel extension 46 engaged within the hub bore 23 and lugs 47 and 48 circumferentially out of register by an amount sufficient to allow axial seating of the gear. The gear is then rotated on the hub to bring gear lugs 47 under rotor lugs 48 in proper circumferential register. The gear will already have its slotted apertures 32, and if the mating dowel bores in the rotor hub have already been provided the dowels are then installed. If the hub has not yet been bored for the dowels the operation may be performed through the slotted apertures 32, which provides a convenient method of locating the mating holes in the rotor hub. The bearing 24 may be positioned within rotor bore 23 either before or after installing the gear, as may be convenient.

In some cases it may be desirable to have the dowels already seated in the rotor hub before assembling the gear to it. In such a case separable rotor lugs 48 may be used, the gear being axially seated with its slotted apertures 32 in engagement with the protruding dow els, and the lugs 48 then attached to the rotor over the gear lugs 47.

What is claimed is:

1. In a rotary mechanism, a rotor and gear assembly comprising in combination a rotor having a rotational axis and parallel sides transverse to the axis and having a bore therethrough coaxial therewith, a ring gear borne at one side of the rotor coaxial with the bore, a plurality of at least three means at selected radial locations securing the gear against rotation relative to the rotor, each of the securing means permitting differential thermal movement between the rotor and gear along a radius passing through its own location but preventing differential movement in other directions and thus maintaining concentricity of the rotor and gear.

2. The combination recited in claim 1, wherein each of the securing means comprises a pin projecting in the axial direction from the gear side of the rotor and tightly seated therein, and the gear has an aperture into which the pin extends, the aperture having a length in the radial direction greater than the diameter of the pin and having a width providing a close fit with the pin in the circumferential direction.

3. The combination recited in claim 2, wherein the plurality of securing pins are circumferentially spaced around the rotor bore and radially outside thereof, the gear has an equal number of apertures spaced congruently therewith, and the pins and slots cooperate to prevent relative rotation and to prevent relative movement in directions transverse to the axis and thus maintain concentricity of the rotor and gear.

4. The combination recited in claim 3, wherein there are at least three such securing means, and at least two of such securing means cooperate to prevent relative movement between the gear and the rotor in any direction transverse to the axis.

5. The combination recited in claim 4, wherein the rotor bore has a bearing positioned therein coaxial with the gear, and the plurality of gear securing means cooperate to allow differential thermal expansion between the rotor and gear without altering concentricity of the gear and bearing and without imposing strain on the parts of the assembly and without restricting thermal growth of the bearing.

6. The combination recited in claim 5, wherein the gear has a barrel extension positioned within one end of the bearing bore in close fitting relationship therewith to establish concentricity of the gear therewith.

7. The combination recited in claim 4, wherein the assembly has axial detent means between the gear and the rotor, the detent means being in firm engagement with the rotor and restricting gear movement only in the axial direction without abutment in the transverse and circumferential directions.

8. The combination recited in claim 7, wherein the gear has an aperture therethrough for each detent means, and each detent means comprises a bolt member seated in the rotor and having a shank portion extending through the gear aperture without contact with the wall thereof, the shank portion having an enlarged head restricting the gear in the axial direction, there being an axial clearance space between the gear and the bolthead when the assembly is in the cold state allowing thermal expansion in the axial direction without imposing strain on the parts of the assembly.

9. The combination recited in claim 8, wherein the number of axial detent means is equal to the number of securing means, and the detent means and the securing means are disposed alternately around the rotor bore and radially outside thereof.

10. The combination recitedin claim 9, wherein the rotor has a fiat-bottomed bore for each detent means with an inwardly extending tapped hole coaxial therewith, the shank portion of the bolt member having a flat bottom and a coaxial threaded portion of smaller diameter extending therefrom, the threaded portion being in interference engagement with the tapped hole and holding the shank portion against the bottom of its bore, the gear aperture for the detent means has a counterbore for the bolthead and of larger diameter than the bolthead, the depth of the bore in the rotor and the length of the shank portion and the depth of the gear counterbore all being proportioned to leave an axial clearance space between the bolthead and the bottom of the counterbore when the assembly is in the cold state.

l]. The combination recited in claim 7, wherein the axial detent means comprises a plurality of lugs extending radially outwardly from the gear, and a like plurality of lugs extending radially inwardly from portions of the rotor and in radial registry with the gear lugs and positioned axially outwardly therefrom to restrict axial movement of the gear.

12. The combination recited in claim 11, wherein there are at least two such axial detent means.

13. The combination recited in claim 12, wherein the lugs extending from the rotor are integral with the material thereof. 

1. In a rotary mechanism, a rotor and gear assembly comprising in combination a rotor having a rotational axis and parallel sides transverse to the axis and having a bore therethrough coaxial therewith, a ring gear borne at one side oF the rotor coaxial with the bore, a plurality of at least three means at selected radial locations securing the gear against rotation relative to the rotor, each of the securing means permitting differential thermal movement between the rotor and gear along a radius passing through its own location but preventing differential movement in other directions and thus maintaining concentricity of the rotor and gear.
 2. The combination recited in claim 1, wherein each of the securing means comprises a pin projecting in the axial direction from the gear side of the rotor and tightly seated therein, and the gear has an aperture into which the pin extends, the aperture having a length in the radial direction greater than the diameter of the pin and having a width providing a close fit with the pin in the circumferential direction.
 3. The combination recited in claim 2, wherein the plurality of securing pins are circumferentially spaced around the rotor bore and radially outside thereof, the gear has an equal number of apertures spaced congruently therewith, and the pins and slots cooperate to prevent relative rotation and to prevent relative movement in directions transverse to the axis and thus maintain concentricity of the rotor and gear.
 4. The combination recited in claim 3, wherein there are at least three such securing means, and at least two of such securing means cooperate to prevent relative movement between the gear and the rotor in any direction transverse to the axis.
 5. The combination recited in claim 4, wherein the rotor bore has a bearing positioned therein coaxial with the gear, and the plurality of gear securing means cooperate to allow differential thermal expansion between the rotor and gear without altering concentricity of the gear and bearing and without imposing strain on the parts of the assembly and without restricting thermal growth of the bearing.
 6. The combination recited in claim 5, wherein the gear has a barrel extension positioned within one end of the bearing bore in close fitting relationship therewith to establish concentricity of the gear therewith.
 7. The combination recited in claim 4, wherein the assembly has axial detent means between the gear and the rotor, the detent means being in firm engagement with the rotor and restricting gear movement only in the axial direction without abutment in the transverse and circumferential directions.
 8. The combination recited in claim 7, wherein the gear has an aperture therethrough for each detent means, and each detent means comprises a bolt member seated in the rotor and having a shank portion extending through the gear aperture without contact with the wall thereof, the shank portion having an enlarged head restricting the gear in the axial direction, there being an axial clearance space between the gear and the bolthead when the assembly is in the cold state allowing thermal expansion in the axial direction without imposing strain on the parts of the assembly.
 9. The combination recited in claim 8, wherein the number of axial detent means is equal to the number of securing means, and the detent means and the securing means are disposed alternately around the rotor bore and radially outside thereof.
 10. The combination recited in claim 9, wherein the rotor has a flat-bottomed bore for each detent means with an inwardly extending tapped hole coaxial therewith, the shank portion of the bolt member having a flat bottom and a coaxial threaded portion of smaller diameter extending therefrom, the threaded portion being in interference engagement with the tapped hole and holding the shank portion against the bottom of its bore, the gear aperture for the detent means has a counterbore for the bolthead and of larger diameter than the bolthead, the depth of the bore in the rotor and the length of the shank portion and the depth of the gear counterbore all being proportioned to leave an axial clearance space between the bolthead and the bottom of the counterbore when the assembly is in the coLd state.
 11. The combination recited in claim 7, wherein the axial detent means comprises a plurality of lugs extending radially outwardly from the gear, and a like plurality of lugs extending radially inwardly from portions of the rotor and in radial registry with the gear lugs and positioned axially outwardly therefrom to restrict axial movement of the gear.
 12. The combination recited in claim 11, wherein there are at least two such axial detent means.
 13. The combination recited in claim 12, wherein the lugs extending from the rotor are integral with the material thereof. 